Chromium manganese - nitrogen bearing stainless alloy having excellent thermal neutron absorption ability

ABSTRACT

This high chromium manganese and nitrogen bearing austenitic stainless alloys have superior strength, high corrosion resistance, excellent weldability and advanced thermal neutron absorption ability. These austenitic iron based stainless alloys have incorporated the neutron absorbing elements as single: gadolinium, hafnium, boron or as a mixture and its in-situ nitrides.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application filed Jan. 20, 2008, and is incorporated herein.

FIELD OF INVENTION

This invention relates generally to the art of austenitic iron based alloys and more particularly to high chromium manganese, nitrogen-bearing alloys having excellent thermal neutron absorption ability. It is used as cast or hot formed components for nuclear fuel transportation cask, spent nuclear fuel storage casks-baskets and as structural material in nuclear industries. These applications are demanding structural material having superior strength, good workability, weldability and long time corrosion resistance.

BACKGROUND OF THE INVENTION

Thermal neutrons are generated and emitted by spent nuclear fuel contained in the containers; nuclear fuel transportation casks, spend nuclear fuel storage casks. For preventing thermal neutrons from initiating an unwanted nuclear chain reaction, such demand has created a need for the materials used for these components to have excellent thermal neutron absorption ability. Furthermore, for preventing such container materials from undergoing damage by environmental corrosion, it is generally demanded that the base metals and weld zones of the material are also corrosion and crack resistance.

Therefore, borated austenitic stainless steel has been developed as structural material for such applications. This is because boron (B) has large absorption cross section for thermal neutrons. However, borated stainless steel has limited usefulness because of known processing drawback, as not sufficient hot/cold workability. Toughness is deteriorated with the increase the boron content to the demanded ranges of about 1% B and the materials can be difficult to weld; cracks are generated at the welded zones.

Excellent castability and as cast superior strength will be required from the alloy to produce by conventional casting method the components for nuclear fuel transportation casks, spend nuclear fuel storage casks. The material for the cask components formed by hot/cold works will require good hot/cold workability and weldability. To achieve the neutron absorption ability of the alleys, mainly gadolinium (Gd) will be incorporated in the austenitic alloys. Gadolinium has neutron absorption ability about 4.4 times as great as that of boron of an identical weight.

For example, in U.S. Pat. No. 3,362,813, a stainless steel alloy containing a minimum of 5% ferrite is disclosed. However, this prior art alloy described has substantial drawback, as hot/cold workability and cracks are generated when the alloy has less than 5% of ferrite. Additionally, in U.S. Pat. No. 6,730,180, an austenitic stainless steel alloy is disclosed. However, some of the ranges of components disclosed with respect to that composition are not within the useful ranges disclosed herein. Thus, nothing in the prior art appears to teach the compositions disclosed herein, particularly with respect to the trice amount of ferrite in the austenitic stainless steel alloys, and with respect to the high Mn and nitrogen contain and alloys superior strength in as cast condition.

SUMMARY OF THE INVENTION

The present invention addresses the problem of casting a metal to sand mold and cast tearing defects and weldability also hot and cold workability. Consequently it can be produced in conventional high grade steel foundries. Furthermore, the aforementioned positive qualities are primarily a chromium content of 11.0 to 48.0% by weight, a manganese content of 0.5 to 30 wt. a nitrogen content of 0.001 to 1.2 wt. %, a carbon content of trace up to 4.9 wt. % and from the group of rare earth elements-Scandium content of 0.0001 to 1.5 wt. %, (portion of Sc may be supplemented by cerium), a boron up to 5 wt. %, a hafnium 0.001 to 12.0 wt. %, a nickel up to 30 wt. %, a titanium up to 3.5 wt. %, a cobalt up to 4.5 wt. %, a silicon up to 4.0 wt. %, a copper up to 6.0 wt. %, a molybdenum up to 6.0 wt. %, a gadolinium up to 15 wt. % and up to about 3 wt. % of each one selected from the group consisting of: zirconium, vanadium, niobium, tantalum, tungsten, calcium, magnesium and rare earth elements with the balance being an iron and other trace elements.

It is generally known that increasing the Cr content is effective to improve corrosion resistance of our presently invented alloy. Our high chromium iron based alloy contains 11% to 48% Cr. The middle range of Cr is 27% to 32%. It is not favorable to have excessively high levels of Cr because this lowers an alloys thermal stability however high levels are required. In efforts to avoid thermal stability issues, the invented alloy has a desirable property in its austenitic structure of upper level of manganese content and nitrogen addition is required to sustain the austenitic structure.

These alloys with high Mn content—8 to 30 wt. % are very susceptible to hot cracking. High levels of Mn content in these alloys causes the alloys to have large dendrites. Sulfur in high chromium manganese alloys combines with manganese to form MnS sulfides. Thus, sulfur appears in the casting or ingots dendrites macrostructure as discrete and randomly distributed large globules inclusions. These large MnS inclusion work as the micro-niches and causes the hot tearing and cracking of the material during hot or cold working or welding. One particular form of defect on the high chromium manganese-nitrogen bearing castable alloys are a crack-like opening called a hot tear. Hot tears occur when alloys with large dendrites are cast under conditions where there is a substantial restraint against uniform shrinkage caused by the mold or the configuration of the article itself. These alloys have high linear shrinkage about 2.2-2.8%. The reduction of temperature during solidification causes sufficient thermal shrinkage and thermal stresses that the partially liquid casting is unable to support the thermal stresses, thereby tearing the casting. In the present approach the cast article is repaired by removing or excising the primary defect and region around the defect by grinding it away. Then the volume metal removed is filled by the filler alloy, preferably having substantially the same composition as that used in the remainder of the article, by a process such as welding. Because the filler alloy has substantially the same composition as the base cast metal of the article, it is also subject to formation of hot tear defects, termed herein-filler defects. We found that small content of the scandium about 0.0001 to 0.05% by wt. significantly restored the tearing resistance and weldability of the present invention alloys.

These alloys contain gadolinium up to 15 wt. %. It may be included others elements such as europium, samarium, dysprosium, boron and hafnium. All these elements have high neutrons absorption cross-section, but the gadolinium has the highest of all equaling 49000 barns. Superior strength, toughness, hot workability, cold workability, tearing resistance, weldability and long term corrosion resistance are most required.

Moreover, boron, hafnium and the rare earth metals such as gadolinium, samarium, and europium have created interest for the application in atomic reactors as control rods because of their high thermal-neutron capture cross section. It is well known that the rare-earth metals form nitrides, carbides. Especially gadolinium and hafnium vigorously reacting with atomic nitrogen highly saturated within the invented alloys and form the highly stable nitrides. These nitrides GdN and HfN, formed while the alloy is still liquid, will be carried over into the slab-ingot or casting and created nucleus of solidification. Therefore, refining the alloy dendrites. Nitrogen is critical alloying element in the steel of the invention. Nitrogen is very strong austenite stabilizer, which affords several advantages. In connection with welding, some alloying elements with poor solubility are strongly segregate. This particularly concerns gadolinium, which exist in high amount in the steel of the invention. In the inter-dendritic regions the gadolinium contents often may be so high that this is causing precipitation of gadolinides-(Ni,Cr)₅Gd, (Fe,Ni)₃Gd and in addition, these may be surrounded by other inter-metallic phases as Laves's phase, sigma-phase, and chi-phase and causing brittleness of this micro-regions. We have found as the nitrogen at high contents essentially delay the precipitation of these phases and diminish its negative effects. Furthermore, nitrogen very strongly increases the pitting and crevice corrosion resistance and it also strongly improves the mechanical strength of the steel, while at the same time maintaining good impact strength and deformability (shapeability).

Gadolinium has a large neutron absorption cross-section but in iron base alloys Gd is essentially insoluble in the austenite-ferrite alloy matrix. However, as a result of investigation on Gd addition on properties of that alloys contain about: 11-32% by wt. Cr and 10-30% by wt. Ni, and 0.5-10% by wt. Mn. and 0.005-15% by wt. Gd. The alloy at over 1% Gd formed an interdendritic (Fe, Ni)₃Gd intermetalic, and the amount of the (Fe, Ni)₃Gd phase increased with increasing Gd concentration. At the same event the alloy matrix was shifting towards ferritic matrix. It is believed that the shift towards the ferritic microstructure was due to extensive nickel depletion from the matrix and contribution of the Ni to the intermetallic-(Fe, Ni)₃Gd increased volume fraction. At this event the alloy was gaining hardness and losing the hot and cold workability. We discovered as small content of the scandium about 0.0001 to 0.3% by wt. substantially improved the desired lost properties as hot and cold workability.

Moreover, we discovered that portion of the nickel depleted from the metal matrix in the invented alloys may be supplemented by addition of about over 10% by wt of manganese and the nitrogen at its solubility ranges. The Mn content of about 12% by wt. and nitrogen content about 0.45% has restored the desirable hot and cold workability and substantially increased strength and toughness of the present invented alloys. The nitrogen was diminishing of the gadolinium compounds of (Fe, Ni)₃Gd by scavenging the Gd and forming GdN and prevent liquation of the gadolinium compounds which was causing cracking during hot forming. In addition, the GdN refine the dendrites and work as neutrons absorbers. Furthermore nitrogen substantially increases the mechanical properties of the alloy by solid solution strengthening effect. During our research work with the steel of this invention we have surprisingly found that the nitrogen austenite stability is so high that the inter-dendritic regions, in spite of the very high contents of gadolinium, will maintain their austenitic microstructure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns high chromium manganese and nitrogen bearing austenitic stainless alloys having superior strength, high corrosion resistance, excellent weldability and advanced thermal neutron absorption ability. This austenitic iron based stainless alloys have incorporated the neutron absorbing elements as single: gadolinium, hafnium, boron or as a mixture and its in-situ nitrides. This structural material is for use in nuclear criticality control applications in the nuclear industry.

Considering all above, a wrought austenitic stainless steel alloy is disclosed comprising in % by weight:

C: less than 0.03%; N: 0.1-0.9%; Cr: 18-28%; Mn: 2.5-16%; Ni: 8-28%; Mo: 0.5-4.5%; Si: less than 0.5%; (d: 0.005-4.5%; Hf: 0.001-0.5%; B: 0.001-0.5%; Sc: 0.0001-0.09%; P: not more than 0.03%; S: not more than 0.02%; Balance is Fe and incidental impurities, wherein the ferrite is less than 5%, where the hot forming range is from about 1300 F-2050 F. By process of hot working can be make plate, sheet, bar and forged or extruded shapes.

In another embodiment N: can be 0.45-0.55%; Cr: 22-26%, Mn: 11-14%, Ni: 12-18%, Mo: 1.5-2.5%, Si: 0.3%, Gd: 1.5-2.8%, Hf: 0.01-0.3%, B: 0.003-0.05%, Sc: 0.01-0.05% P: not more than 0.03%, S: not more than 0.02%, Balance is Fe and incidental impurities, wherein the ferrite is less than 2%, where the hot forming range is from about 1800 F-2050 F. By process of hot working can be make plate, sheet, bar and forged or extruded shapes.

As an alternative embodiment conventional castable austenitic stainless alloy is disclosed comprising in % by weight:

C: less than 0.8%; N: 0.01-0.7%; Cr: 26-34%; Mn: 4.5-18%; Ni: 12-28%; Mo: 0.5-4.5%; Si: less than 1%; Gd: 0.005-6.5%; Hf: 0.001-1.5%; B: 0.001-0.3%; Sc: 0.0001-0.09%; P: not more than 0.05%; S: not more than 0.04%; and up to about 3 wt. % of each one selected from the group consisting of: zirconium, vanadium, niobium, tantalum, tungsten, calcium, magnesium and rare earth elements with the balance being an iron and other trace elements. Wherein the ferrite is near 0%, where the material can be formed by conventional casting methods as: sand-cast, investment-cast, centrifugally-cast, and produce components for nuclear fuel transportation cask, spend nuclear fuel storage casks-baskets, tubes, pipes and use as structural material in nuclear industries.

EXAMPLES

The following examples should not be considered as limitations of the present invention, but are merely intended to teach how to make the alloys based upon presented experimental data.

High efficiency centrifugal pump cast impellers are consider a difficult casting to produce without hot tears defect. The defects are apparent at the areas that join the impeller shrouds and the vanes. To reduce the tears at these areas it must be made by supporting-reinforcing fines trough the lengths of the vanes, then after casting is cleaned these fines must be removed, the hot tears defects welded and grinded off to the original radius. This may take several hours of hand grinding of the cast impeller one vane. These kinds of sand molds were prepared for casting—without supporting fines.

Our alloys may be melted using conventional electric induction furnace with refractory lining suitable for melting steel or stainless steel alloys. Moreover, the high chromium and manganese content alloys are easy to oxidize during melting. It is beneficial to use protective argon blanketing process during melting and pouring of these alloys. Reactive metal elements such as hafnium, scandium, gadolinium and others must be added last and only after the melt has been sufficiently deoxidized. Scandium may be added to the melt as an aluminum-scandium alloy or to the ladle during the tapping.

Our invention alloys:

Alloys Cr Mn N Ni Mo Hf Sc Co B C Gd Alloy 1 28 16 0.72 14 2.6 0.3 0.05 0.01 0.005 0.22 0.002 Alloy 2 24 14 0.40 12 2.2 0.01 0.09 0 0.003 0.09 2.8

Balance is substantially Fe and residual amounts P,S,Si

The first alloy was casted by producing the impeller mentioned above and additional tensile test bars. Alloy number 2 was casted on the same impeller as number 1 and forging ingots for further forging. Both alloys exhibited desired production processing properties.

From these heats the tensile tests bars was solution annealed at 2050 F and machined to a 0.5 inch gauge and mechanical properties tested:

The mechanical test results:

Tensile strength Elongation Alloy Yield strength (ksi) (ksi) (%) Alloy 1: as cast and 98.5 128.8 19.8 solution annealed Alloy 1: forged and 119.5 145.7 28.9 Solution annealed Alloy 2: as cast and 61.7 81.3 31.9 solution annealed Alloy 2: forged and 87.2 128.1 37.4 Solution annealed

While the present invention has been disclosed and described, it is to be understood that this invention is not limited to the particular configurations, process steps and materials disclosed herein as these may vary to some degree. Any person skilled in the art will appreciate that many modifications, changes, and substitutions can be made without departing from the spirit of the invention. The invention will be limited only by the appended claims and equivalents thereof. 

1. Austenitic stainless steel having excellent thermal neutron absorption ability, comprising of the following chemical composition on the weight % basis: gadolinium at from about 0.001% to 15%; chromium at from about 11% to 48%; manganese at from about 0.5% to 30%; nitrogen at from about 0.001% to 1.2%; carbon at from about 0.001% to 2.2%; scandium at from about 0.0001% to 1.5%; boron 0% to about 5%; hafnium at from about 0.001 to 12%; nickel at from about 4% to 30%; silicon 0% to about 4%; copper 0% to about 6%; molybdenum 0% to about 6%, said alloy further comprising 0 to 3% of each of one or more of zirconium, vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum cobalt, calcium , magnesium and rare-earth elements, the balance comprising iron and inevitable impurities.
 2. The alloy of claim 1, wherein the alloy comprises at least one of molybdenum, silicon, copper, boron, each in an amount of at least 0.001% by weight.
 3. Alloy of claim 1, wherein the alloy comprises 18% to 28% by weight of chromium and the molybdenum comprises 0.5% to 4.5% by weight and manganese comprises 4.5% to 12% by weight, nitrogen comprises 0.3% to 0.55% by weight, gadolinium comprises 0.1% to 4.5% by weight, nickel comprises 8% to 16% by weight.
 4. The alloy of claim 3 wherein comprises hafnium 0.01 to 0.3% by weight, scandium comprise 0.001% to 0.05% by weight, carbon comprises less than 0.04% by weight and is configured as canister.
 5. Alloy of claim 1 wherein the alloy comprises chromium 20% to 26% by weight, molybdenum 1.5% to 3.5% by weight, manganese 11% to 13%, nitrogen 0.45% to 65% by weight, gadolinium 1.5% to 3%, nickel 10% to 14% by weight.
 6. The alloy of claim 5 wherein the alloy comprise boron 0.005% to 0.5% by weight, scandium 0.001% to 0.09% by weight, carbon less than 0.03% by weight.
 7. The alloy of claim 6 wherein the alloy comprises hafnium 0.001% to 0.5% by weight and is configured as canister-basket interior. 